An attempt to use the same transaction output in more than one conflicting transaction.
Double-spend meaning
Bitcoin prevents confirmed outputs from being spent twice under one accepted history. A transaction input points to an earlier unspent output. Once a valid chain spends that output, another transaction trying to spend it conflicts.
How it works in Bitcoin
Before confirmation, different nodes may temporarily know about different conflicting transactions. Miners choose which valid candidate to include, subject to their policies. Once one confirms, the competing spend is invalid on that chain.
A simple example
A buyer broadcasts one payment to a shop and a conflicting payment back to themselves. If the shop releases valuable goods before confirmation, it must judge the risk that the second transaction confirms instead.
Why Double-spend matters
Transaction terms matter whenever bitcoin moves. Understanding them reduces mistakes involving fees, confirmation status, change outputs and irreversible destinations.
What people often get wrong
A duplicate wallet display or a delayed explorer is not itself a double-spend. The key issue is whether two transactions consume the same previous output.
Quick checks before using the term
- Is the transaction only broadcast or already confirmed?
- Which outputs and fee are being created?
- Can wallet policy replace or delay it?
Apply these questions to the concrete example above. A precise answer is more useful than repeating the definition without naming the actor, rule or failure path.
Check the primary reference
This entry is written in plain English, but the technical source is available when you need exact protocol detail.
Bitcoin Developer Guide: Transactions ↗